The present invention relates to transmission fluids, especially automatic transmission fluids, which contain a phthalic acid to improve the corrosions performance thereof. It also relates to a method for providing phthalic acids, especially terephthalic acid, in a soluble form suitable for such use.
In recent years, particularly in Europe, corrosion of copper alloys by automatic transmission fluid has become a subject of increasing concern. This is due in part to the introduction of electrical components and controls which may be affected by otherwise minor corrosion of wires, contacts, and the like. Bushing wear and the possibility of catalysis of transmission fluid oxidation, due to the presence of leached metals, are also of concern. To address these problems, various anticorrosion approaches and materials are known.
U.S. Pat. No. 4,801,729, Holstedt et al., Jan. 31, 1989, discloses lubricating compositions containing a boron-containing heterocyclic compound, as well as anti-wear and friction-reducing compounds, corrosion inhibitors and oxidation inhibitors. A small amount of terephthalic acid is effective as a lead corrosion inhibitor in the composition. Additional components can include antiwear additives such as zinc dithiophosphate, triphenyl phosphorothionate, etc. Automatic transmission fluids are disclosed.
U.S. Pat. No. 4,629,579, Jessup et al., Dec. 16, 1986, discloses boron derivatives used in lubricating oils. Terephthalic acid is included in a lubricant composition as a corrosion inhibitor.
U.S. Pat. No. 3,692,681 discloses a dispersion of terephthalic acid in a hydrocarbon oil medium which contains highly hindered acylated alkylene polyamines, that is, a highly hindered basic amine containing ashless detergent. The terephthalic acid is dissolved in an auxiliary solvent (a tertiary alkanol or DMSO) and the solution mixed with a hydrocarbon oil solution of the ashless detergent and the auxiliary solvent separated by vaporization or other convenient means.
U.S. Pat. No. 2,900,339, Lowe, Aug. 18, 1959, discloses a process for preparing lubricant compositions and concentrates. Stable dispersions of phthalic acids in oils can be prepared by reacting a phthalic acid with a tertiary amine, combining the reaction mixture thus obtained with an oil of lubricating viscosity, and heating the combined oil and reaction mixture to liberate the tertiary amine.
U.S. Pat. No. 2,809,160, Steward et al., Oct. 8, 1957, discloses a lubricating oil composition having improved corrosion inhibiting properties by using a phthalic acid (isophthalic acid or terephthalic acid). A lubricating oil detergent additive is present in a minor amount, sufficient to suspend the phthalic acid in the oil.
Phthalic acids, and in particular terephthalic acid, although known as a corrosion inhibitor, are generally insoluble or difficultly soluble in oil media. The present invention, therefore, solves the problems of providing phthalic acids in a soluble form in lubricant media, and using the resulting compositions to impart corrosion resistance especially to automatic transmission fluids.